1. Field of The Invention
This invention relates to the field of gaskets which are used in joints of lined vessels and conduits, which linings are non-machined non-metallic, non-corrosive materials such as glass and which require gaskets to form a seal between joints in these vessels and conduits without breaking the brittle lining while at the same time the gasket and brittle joint lining are subjected to high temperatures and high pressure-temperature coefficients.
2. Definition Used Herein
Vessel as used herein is used to include those conduits such as pipes through which liquids can flow as well as those containers in which liquid is confined for some period of time.
3. Discussion of Prior Art
There are many varied types of metal vessels used in chemical and food processes and the like and in the transportation of fluids. Many of these vessels require linings of material which are resistant to corrosion of the fluids these vessels contain or transport. Examples of vessels which are often so lined are reactors, columns, mixers, pipelines, storage tanks, pipe valves, evaporators, dryers, blenders, and the like.
Necessarily these vessels often are in sections, and thus have connecting joints such as flanged joints to connect these sections. For example, flanged pipes and Pfaudler kettles are examples of such vessels having flanged joints.
Most often these vessels are made of metal in industrial sized application. This is true even when the liquid which the vessel is to contain, or is to transfer, is corrosive with respect to the metal. To avoid this corrosion these vessels are often lined with non-corrosive linings. Economics usually dictate that these linings be made of the cheapest materials available which will serve the purpose at hand. These cheaper, non-corrosive materials most often turn out to be non-metallic materials as opposed to the non-corrosive precious metals which are very expensive. Examples of these non-metallic lining materials are glass, ceramic, enamel, fused silica, fused quartz, boro-silicates, Herisite, stoneware, and the like which are well known to those skilled in the art of vessel construction.
These relatively cheaper lining materials, however, are all quite brittle when compared to the metal walls of the vessels of which they are used to line. This brittleness turns out to be a quite complicating factor when determining what gaskets can be used in the joints when the different sections of the vessel are connected together such as by latches or flange bolts. This complication arises from the necessity of choosing a gasket which is not only resistant to the corrosive liquid in the vessel, but is also at the same time both (a) sufficiently soft so as not to break the brittle linings on the vessel joints seating surfaces, and (b) sufficiently resistant to gasket cold flow at the temperature at which the vessel is to operate so as not to loose the seal it is intended to provide.
A further complicating factor arises from the fact that these linings are very seldom machined so as to provide a flat surface on the flange seating surfaces. For example, glass linings are usually left in what is referred to in the trade as an "as-fired" condition. The cost of machining such surfaces into a flattened condition is quite high. It is known that when non-flat surfaces, that is non-machined surfaces, are subjected to a compressive load such as when opposing flange faces are bolted tight, then this load is unevenly distributed across the non-flat surfaces with the higher portions of the surface taking much more of the load than the lower portions. When non-brittle or soft surfaces are used, then the uneven surface is known to deform toward an even surface so as to spread this load more uniformly. With brittle surfaces, however, very little of this deformation occurs resulting in surface breakage. Thus, here again the brittleness of these non-metallic lining materials works against the capability of tightly squeezing together flanged joints which are lined with such materials.
A caveat should be made here with respect to glass, ceramics, and other glass-like materials used for such linings. This caveat is that although these materials form a smooth surface in their as-fired state, they very seldom have a flat surface. Rather they have an undulating or wave-like surface which suffers the same load maldistribution problems as do other brittle, non-machined (i.e. not flattened) surfaces.
Engineers and gasket manufacturers have developed different materials and rating systems for gaskets to be used in sealing joints of vessels lined with these brittle, non-metallic linings. Examples of such gasket materials are blue (soft) asbestos, SBR (synthetic butyl rubbers), natural rubber, neoprene elastomers, nitrile rubbers, cellulose fibers, cork, and various kinds of plastics such as the TEFLON (tetrafluoroethylene fluorocarbon) polymers). (TEFLON is a trademark of the E. I. DuPont de Nemeurs and Company of Delaware). These gasket materials are often used in combination with one another and with various kinds of binders to achieve the seal required for the specific task at hand. Gaskets of different layers of these materials are also used. However, for the brittle lined vessels, one common thread of gasket design is present, and that is that the gaskets be sufficiently flexible and soft to sufficiently be compressed between the brittle, uneven, wavy lined seating surfaces of the vessel joints to form a seal without breaking the brittle lining on these seating surfaces.
Heretofore such a seal has been accomplished by concentrating on the surfaces of the gasket which contacted these brittle linings of the vessel joint seating surfaces. These surface materials have been chosen to be soft enough to accomodate the brittle lining on the opposed seating surfaces of the joint flanges when subjected to the seating stress pressures required to make a seal between the flanges.
One problem with this approach, however, is that the softer the gasket material is, the more prone it is to "cold flow" over a period of time and lose its seal. This is particularly true for such gasket materials as the operating temperature to which the gasket is exposed is increased. The material becomes hot enough to slowly flow in and from the joint during "cold flow".
To help understand the approach which design engineers have taken in the past to solve the problems associated with making these gaskets, reference is made to FIGS. 3 and 4. Therein is disclosed a top view (FIG. 3) and a side sectional view (FIG. 4) taken along the line 4--4 in FIG. 3 of a typical prior art gasket 40. The inner gasket body is made of a layer 42 of a soft gasket substance such as blue asbestos as a filler for the outer envelope 44 of the gasket body. More than one layer is sometimes used to form this inner gasket body 42. Be that as it may, however virtually all of such gaskets involving higher temperature and pressure conditions employ a softer envelope 44 to do several things. This envelope is made of a soft material such as Teflon so as: (1) to be soft enough to conform to the uneven gasket surface, and (2) to have a high enough melting point to avoid cold flow. To avoid leakage between the asbestos filler 42 and the Teflon layers on either side of it, the outer Teflon gasket body is formed in a shape so that the inner surface 46 of the gasket assembly 40 is a continuous Teflon wall. Thus the only surface presented to the corrosive fluid inside the gasket is the inner wall 46 of the Teflon envelope. Teflon is soft enough and has sufficiently superior corrosion resistant and cold flow resistant properties to be quite useful in most applications. But what happens when the vessel operating temperature has to be so hot that the soft Teflon envelope cold flows until the seal is lost? The present invention provides an economical gasket assembly whose design is such that it can be used in lined joints operating at temperatures above the maximum temperature rating of the soft envelope materials such as Teflon.
Besides maximum temperature, a second rating by which gaskets are measured is their pressure-temperature coefficient. This coefficient is the maximum mathematical product of the temperature at which the gasket is expected to function and the pressure of the fluid in the vessel tending to push the gasket out of the vessel joint. If the vessel design parameters call for a gasket which exceeds either its maximum cold-flow temperature rating or its pressure-temperature coefficient rating, then another gasket material has to be found. But as stated above the soft gasket materials normally used have low temperature ratings. They also have low ratings. They also have low pressure-temperature coefficient ratings. But yet again, many process efficiency improvements are calling for higher and higher temperatures and pressures.
It would, therefore, be advantageous to have a gasket which could meet these higher temperature and pressure requirements while still using relatively cheap gasket materials so that the relatively cheap non-metallic, non-corrosive linings can be used.
The present invention provides such a gasket by changing the design concept of such gaskets. It discards the prevailing concept that the gasket seating surface themselves must be the only part of the gasket that is so soft so as to conform to the brittle, non-flat seating surfaces of the joint. Rather it provides the flexibility required for not breaking the brittle lining, but still sealing such linings by use of a plurality of corrugated metal sections alternating with sections of gasket filler material in the interior of the gasket. The very low modulus of elasticity or softness once thought necessary for the gasket seating faces themselves to have is replaced with the concept of raising the modulus of elasticity of these seating surfaces but decreasing the modulus of elasticity of the whole gasket assembly. The harder gasket seating surfaces increases the temperature at which the vessel can operate with the gasket maintaining its seal.